The invention relates to a display device comprising a light guide, a movable element and selection means to locally bring said movable element into contact with the light guide, said selection means comprising row and column electrodes and means for applying addressing voltages to the row and column electrodes.
A display device of the type mentioned in the opening paragraph is known from U.S. Pat. No. 4,113,360.
In said patent, a description is given of a display device comprising a first plate of a fluorescent material, in which, in operation, light is generated and trapped (so that this plate forms a light guide), a second plate which is situated at some distance from the first plate and, between said two plates, a movable element in the form of a membrane. By applying voltages to addressable electrodes on the first and second plates and to electrodes on the movable element, the movable element can be locally brought into contact with the first plate, or the contact can be interrupted. A transparent contact liquid is present on the contact surfaces. At locations where the movable element is in contact with the first plate, light is decoupled from said first plate. This enables an image to be represented. If the movable element is not in contact with the light guide, it is in contact with the second plate.
For the proper functioning of the display device, it is important that, on the one hand, the contact between the light guide and the movable element can be brought about and interrupted in an accurate and reliable manner, but that, on the other hand, the design is simple and does not require much energy to operate.
It is an object of the invention to provide a display device of the type mentioned in the opening paragraph, which provides a simple and yet reliable device.
To achieve this, the display device in accordance with the invention is characterized in that the selection means comprise means for applying voltages to the electrodes in dependence on a previously applied voltage or voltages on the electrodes.
In the known device, the position of the movable element, i.e. whether or not it makes contact with the light guide is dependent on the applied voltages, and on said voltages only. The inventors have realized that the fact whether or not the movable element moves is dependent on the forces acting on the element. The forces acting on a movable element are not only dependent on the applied voltages, but also on other forces acting on the element and on its position vis-xc3xa1-vis the electrodes. Said position is also dependent on the history of the element, i.e. previously applied voltages and position. The electric forces acting on the movable element are non-linearly dependent on the distances between the movable element and the electrodes. Because of the non-linear relationship between force and distance, the device exhibits a memory effect. When the movable element is near one of the electrodes, only a relatively large voltage difference between the electrodes can move the element to the other electrode. This, however, also means that once a movable element is in a certain position, it will stay in such a position, even if the voltages applied are changed, provided that they do not change to such a large degree that the movable element is moved to the other electrode. Since the device exhibits a xe2x80x98memory effectxe2x80x99, i.e. it is not only the momentary voltages applied which determine whether or not the movable element moves, but this is also determined by previously applied voltages. Using this insight, one or a number of advantages can be obtained. The device can be simplified, and/or the addressing voltages applied to the device can be simplified and/or the energy required can be lowered and/or the reliability of the device can be increased. Also grey levels can be made, as will be explained.
A preferred embodiment of the device in accordance with the invention is characterized in that the means for applying addressing voltages apply, in operation, a first set of voltages having a lower and an upper value to a row electrode, and a second set of voltages having a lower and an upper value to a column electrode crossing the row electrode at a crossing area, the device being arranged in such a way that only simultaneous application of a lower value to the row electrode and an upper value to the column electrode, or vice-versa, changes the position of the movable element at the crossing area.
Alternatively, a preferred embodiment of the device in accordance with the invention is characterized in that the means for applying voltages apply, in operation, a first set of voltages having a lower and an upper value to a column electrode, and a second set of voltages having a lower and an upper value to a row electrode crossing the column electrode at a crossing area, the device being arranged in such a way that only simultaneous application of a lower value to the column electrode and an upper value to the row electrode, or vice-versa, changes the position of the movable element at the crossing area.
In these embodiments, application of an upper or lower value on one electrode (row or column) alone does not actuate the movable element at the crossing area of the relevant row and column electrodes. Only simultaneous application of a lower value on one of the electrodes, and an upper value on the other, or vice-versa, will actuate the element at the crossing area. Actuating the movable elements becomes very reliable by this measure. Small deviations of applied voltages do not inadvertently switch an element. Basically, only simultaneous application of two xe2x80x98onxe2x80x99 signals on row and column electrode(s) will turn a pixel xe2x80x98onxe2x80x99 when it is xe2x80x98offxe2x80x99, and simultaneous application of two xe2x80x98offxe2x80x99 signals on row and column electrode(s) will turn a pixel xe2x80x98offxe2x80x99 when it is xe2x80x98onxe2x80x99, as will be further explained in the description.
Preferably, the means for applying voltages apply, in operation, a turn-on addressing voltage to a row electrode, while simultaneously applying addressing voltages to a number of column electrodes crossing said first electrodes to bring the movable element in contact with the light guide at selected crossing areas of the row electrode, and subsequently apply said turn-on addressing voltage to a second row electrode while, simultaneously applying addressing voltages to a number of column electrodes crossing said first and second row electrodes to bring the movable element in contact with the light guide at selected crossing areas of the second electrode, the voltage at the first row electrode being in between the turn-on addressing voltage and a turn-off addressing voltage, such that the position of the movable elements at the crossing areas of the first row electrode does not change.
Alternatively, the means for applying voltages apply, in operation, a turn-on addressing voltage to a first column electrode, while simultaneously applying addressing voltages to a number of row electrodes crossing said first column electrode to bring the movable element into contact with the light guide, at selected crossing areas of the first column electrode and subsequently apply said turn-on voltage to a second column electrode, while simultaneously applying voltages to a number of row electrodes crossing said first and second column electrode to bring the movable element into contact with the light guide at selected crossing areas of the second column electrode, the voltage at the first column electrode being in between the turn-on addressing voltage and a turn-off addressing voltage, such that the position of the movable elements at the crossing areas of the first row electrode does not change.
A turn-on addressing voltage is understood to mean a voltage value which, when combined with a given (turn-on) voltage at a crossing electrode, results in bringing the movable element into contact with the light guide at the crossing area.
Likewise, a turn-off addressing voltage is understood to mean a voltage value which, when combined with a given turn-off voltage at a crossing electrode, results in releasing the movable element from the light guide at the crossing area.
These embodiments are based on the following recognition. When the first row or column electrode is supplied with an xe2x80x98onxe2x80x99 signal (turn-on voltage), and a set of crossing electrodes is supplied with xe2x80x98onxe2x80x99 and xe2x80x98offxe2x80x99 signals (xe2x80x98offxe2x80x99 meaning xe2x80x98not onxe2x80x99), only those pixels corresponding to areas where electrodes cross and both carry xe2x80x98onxe2x80x99 signals will be turned xe2x80x98onxe2x80x99. A first line of picture elements is thus formed.
This step is thereafter repeated for the second (row or column) electrode to form a line of picture elements. However, the voltage at the first row or column electrode is brought to a value between the xe2x80x98onxe2x80x99 and an xe2x80x98offxe2x80x99 value. This means that the first line of picture elements remains visible, i.e. xe2x80x98onxe2x80x99 and the information in said line is preserved. In its simplest form, two lines of picture elements are formed in this manner. It will be clear that this scheme can be expanded to more than 2 lines.
The great advantage is that, while the second (or third etc.) line of picture elements is formed, the first (second etc.) line of picture elements remains xe2x80x98onxe2x80x99. The total intensity of the light is thereby increased substantially in comparison with arrangements in which (as, for instance, in classical CRTs) only one line of picture elements (or pixels) is activated (xe2x80x98onxe2x80x99) at any one time.
This allows multi-line operation, i.e. more than one line (multi-line) is simultaneously active. The lines of picture elements (the video information) could be written in columns or rows. This also allows grey levels to be made.
Preferably, the means for selection supply, in operation, such a sequence of voltages that the percentage of time during which a given row or column electrode is active is approximately (within roughly 50%) uniform for all row or column electrodes, but does show a variation over the device, depending on the distance between the row or column electrode and a nearest light input for the light guide, while the percentage of time during which a given row or column electrodes is active increases as the distance to a nearest light input increases. Light guides show absorption of light. This will cause a reduced uniformity of the light emitted by the display. By increasing the percentage of time during which a row or column electrode is active, this effect can be counteracted to increase the uniformity in the image displayed by the device.
A row or column electrode is active between the time when a turn-on voltage has been supplied to the row or column electrode until a turn-off voltage has been supplied to said row or column electrode.